Coriolis and rotating buoyancy effect on detailed heat transfer distributions in a two-pass square channel roughened by 45° ribs at high rotation numbers

Heat transfer measurements from a rotating two-pass square channel with two opposite leading and trailing walls roughened by 45° parallel ribs arranged in the staggered manner are performed to examine the effects of Reynolds (Re), rotation (Ro) and buoyancy (Bu) numbers on local and area-averaged Nusselt numbers (Nu and $\overline{\mathit{Nu}}$). Full-field Nu distributions over the two rib-roughened leading and trailing walls are measured at the conditions of 4000 ? Re ? 16,000, 0 ? Ro ? 0.8 and 0.0015 ? Bu ? 0.93 (0.05 ? Δρ/ρ ? 0.1) using the infrared thermography which allows for the detailed examination of the Coriolis and rotating buoyancy effects on Nu distributions over the rotating ribbed surface. Selected heat transfer data in term of Nu ratio between rotating and stationary levels illustrates the influences of rotation on local and area-averaged heat transfer performances. Area-averaged $\overline{\mathit{Nu}}$ for the turn region and the inlet and outlet ribbed legs of the rotating two-pass channel are parametrically analyzed to devise a set of empirical heat transfer correlations that permits the evaluation of the interdependent and individual effects of Re, Ro and Bu on $\overline{\mathit{Nu}}$.     

Transport equations for reaction rate in laminar and turbulent premixed flames characterized by non-unity Lewis number

Transport equations for (i) the rate $W$ of product creation and (ii) its Favre-averaged value $\stackrel{?}{W}$ are derived from the first principles by assuming that $W$ depends solely on the temperature and mass fraction of a deficient reactant in a premixed turbulent flame characterized by the Lewis number $Le$ different from unity. The right hand side of the transport equation for $\stackrel{?}{W}$ involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum $\overline{T_{\text{Σ}}}$ of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar premixed flames characterized by $Le<1$ are performed and the linear relation between $T_{\text{Σ}}$ integrated along the normal to a laminar flame and a product of (i) the consumption velocity $u_c$ and (ii) the stretch rate ${\dot{s}}_w$ evaluated in the flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of $\overline{T_{\text{Σ}}}\propto \overline{\rho W\dot{s}}$ is hypothesized, where $\rho$ is the density and $\dot{s}$ is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, premixed turbulent flames in the case of a single-step chemistry and $Le=0.34$, 0.6, or 0.8. In all three cases, the agreement between $\overline{T_{\text{Σ}}}$ and $\overline{\rho W\dot{s}}$ extracted from the DNS is good with exception of large ($\overline{c}>0.4$) values of the mean combustion progress variable $\overline{c}$ in the case of $Le=0.34$. The developed linear relation between $\overline{T_{\text{Σ}}}$ and $\overline{\rho W\dot{s}}$ helps to understand why the leading edge of a premixed turbulent flame brush can control its speed.     

High-pressure burning velocities measurements for centrally-ignited premixed methane/air flames interacting with intense near-isotropic turbulence at constant Reynolds numbers

This paper measures high-pressure turbulent burning velocities (S_T) of lean methane spherical flames at constant turbulent Reynolds numbers (Re_T ≡ u′L_I/ν), where u′ and L_I are the r.m.s. turbulent fluctuation velocity and the integral length scale of turbulence and ν is the kinematic viscosity of reactants. This is achieved by adopting a recently-built double-chamber, fan-stirred cruciform burner with perforated plates that can be used to generate intense near-isotropic turbulence with negligible mean velocities while controlling the product of u′L_I in proportion to the decreasing ν at elevated pressure (p) up to 1.2 MPa. Results show that when Re_T is fixed ranging from 6700 to 14,200, values of S_T decrease similarly as laminar burning velocities (S_L) with increasing p in minus exponential manners, revealing a global response of burning velocities to pressure. In general, the higher Re_T, the higher S_T/S_L at any fixed p. It is found that the curves of S_T/S_L as a function of u′/S_L all exhibit very strong bending under constant Re_T conditions. These results not only reveal that the important effect of Re_T on high-pressure S_T/S_L enhancement, but also suggest that recent findings related with the promotion effect of increasing pressure on S_T primarily due to the enhancement of flame instabilities via the thinner flame without any discussion on the influence of Re_T elevation at elevated pressure should be reconsidered. Moreover, we found that the modified values of S_T at mean progress variable $\overline{c}$ ≈ 0.5 show good agreements between Bunsen-type and spherical flames, suggesting that S_T determined at flame surfaces with $\overline{c}$ = 0.5 may be a better representative of itself regardless of the flame geometries. Finally, various general correlations of $S_{\text{T,}\overline{c}=0.5}$ are compared and discussed. It is found that the present scattering data under different p and Re_T conditions can be merged onto a single curve of ($S_{\text{T,}\overline{c}=0.5}$ ? S_L)/u′ = 0.14Da^0.47, where Da is the turbulent Damköhler number.     

Heat transfer and pressure drop in furrowed channels with transverse and skewed sinusoidal wavy walls

This comparative study examines the detailed Nusselt number (Nu) distributions, pressure drop coefficients (f) and thermal performance factors (η) for two furrowed rectangular channels with transverse and skewed sinusoidal wavy walls. Detailed heat transfer measurements over these transverse and skewed sinusoidal wavy walls at the Reynolds numbers (Re) = 1000, 1500, 2000, 5000, 10,000, 15,000, 20,000, 25,000 and 30,000 are performed using the steady-state infrared thermo-graphic method. Impacts of Re on Nu and f for two tested furrowed channels with transverse and skewed waviness are individually examined. In addition to the macroscopic mixing between the near-wall recirculations and core flows due to the shear layer instabilities in each wavy channel, the secondary flows tripped by the skewed wall-waves further elevate heat transfer performances and distinguish their Nu distributions from those over the transverse wavy wall. The area-averaged Nusselt numbers ($\overline{\mathit{Nu}}$) for two tested furrowed channels with transverse and skewed waviness with 5000 < Re < 30000 fall, respectively, in the ranges of 3.45–3.71 and 3.98–4.2 times of the Dittus–Boelter levels. A set of $\overline{\mathit{Nu}}$ and f correlations for each tested furrowed channel is individually derived using Re as the controlling parameter. By way of comparing the thermal performance factors (η) with a selection of rib-roughened channels, the η factors for the present skewed wavy channel are compatible with those in the channel roughened by the compound V-ribs and deepened scales due to the relative low pressure drop penalties with the equivalent heat transfer augmentations to those offered by V-ribs.     

Numerical study of microchannel heat sink performance using nanofluids

This study presents the numerical simulation of three-dimensional incompressible steady and laminar and turbulent fluid flow of a trapezoidal micro-channel heat sink (MCHS) using CuO/water nanofluid as a cooling fluid. Navier–Stokes equations with conjugate energy equation are discretized by the finite-volume method. CFD predictions of laminar and turbulent forced convection of CuO/water nanofluids by single-phase and two-phase models (mixture model) are compared. The parameters studied include the particle volume fraction (ϕ = 0.204 %, 0.256%, 0.294% and 0.4%), and the volumetric flow rate ($\dot{\mathit{V}}=10\mathrm{mL}/min$, 15 mL/min and 20 mL/min). Comparisons of the thermal resistance predicted by the single-phase and two-phase models with corresponding experimental results show that the two-phase model is more accurate than the single-phase model. In the laminar flow, the thermal resistance of nanofluids is smaller than that of the water, which decreases as the particle volume fraction and the volumetric flow rate increase. In addition, the pressure drop of both nanofluid-cooled MCHS and pure water-cooled MCHS is discussed. For the laminar flow case, the pressure drop increases slightly for nanofluid-cooled MCHS.